Known in the art are apparatus for studying physical properties of industrial aerosols and pulverulent materials functioning on the basis of preliminarily charging particles, with subsequent handling of electrical signals obtained in measuring charges of the particles (cf. USSR Inventor's Certificate No. 372483, Cl. G 01 N 15/02, 1973; U.S. Pat. No. 3,718,029, Cl. 73-28, 1973).
Such apparatus comprises a gas duct for forming a flow of suspended particles, a charging device in the form of an electrode generating a corona discharge wherein particles acquire a charge which is proportional to their surface area, and a device for measuring the total charge of the particles.
The disadvantage of such apparatus resides in that it can mainly measure the concentration of particles of industrial aerosols or pulverulent materials per unit of volume. The application of such apparatus for the analysis of grain-size composition is rather difficult.
In addition, the analysis of grain-size composition of a powder in prior art apparatus is performed during a time period between several hours and several days which makes the production process requiring such an analysis very complicated.
Known in the art is an apparatus for measuring the grain-size composition of powders, comprising a device for transforming a powder into a gaseous flow of individual particles (a powder sample atomizer), an inlet pipe connected thereto and having a narrow tube for forming a jet of aerosol, and a filter for cleaning the gas and for laminarizing the flow.
The apparatus also comprises a charging chamber communicating with the inlet pipe, a precipitating capacitor in the form of a U-shaped pipe, and a device for measuring the total charge of individual fractions. This device has inductive sensors of charged particles, the number of the sensors being equal to the number of fractions, each sensor being installed in a collector plate of the precipitating capacitor, and an electrometric amplifier having an input to which are connected said inductive sensors. A pulse train characterizing the grain-size composition of the powder under study appears at the output of the amplifier (cf. USSR Inventor's Certificate No. 530229, Cl, G 01 N 15/00, 1977). The prior art apparatus also has an air blower, a device for neutralizing static charges and high-voltage power supply sources connected to the charging chamber and precipitating capacitor.
A sample of the powder under study is transformed into a gas flow of individual particles and is admitted to the charging chamber through a narrow tube of an inlet pipe in the form of an aerosol jet. In the charging chamber with an unipolar space charge the particles are charged, the particles of the same size acquiring equal charges. The flow of charged particles is then fed to the precipitating capacitor in which the dispersed phase of the aerosol is distributed in the space into fractions under the action of forces of the electrostatic field, the fractions precipitating at different distances from the point of entry of the flow to the precipitating capacitor. The sensors generate signals, each signal corresponding to the value of charge of particles of a given fraction so as to characterize the size of the particles. The signals are fed to the electrometric amplifier having an output at which a pulse train appears which characterizes the grain-size composition of the powder under study. After the signal is recorded, dust is removed and static charges are neutralized.
The disadvantage of this apparatus resides in that the direct connection of the inductive sensors to the electrometric amplifier results in a substantial leakage of charge containing information on a given fraction which is obtained from the inductive sensor.
This is explained by the fact that the basic condition according to which the time t of measurement of the grain-size composition should be much shorter than the time constant .tau. of the inductive sensor during which the value of the total charge of the particles remains unchanged, that is t&lt;&lt;.tau., is not fulfilled. As the time constant of the inductive sensor is determined by the formula .tau.=RC, wherein R is the input resistance of the amplifier and C is the capacitance of the sensor, it is obvious that the required value of the time constant .tau. of the inductive sensor can only be obtained by making a compromise between the two parameters R and C.
Generally for a majority of apparatus of this type the time t of measurement of the grain-size composition is longer than 100 s. At the same time, the value of the sensor capacitance is determined by the relationship U=Q/C, wherein Q is the total charge of particles of a given fraction, and to achieve the required voltage level U obtained at the sensor, the value of the sensor capacity C should be maximum 100 pF.
Therefore, in order to comply with the requirement that t&lt;&lt;.tau., it is necessary that the input resistance of the amplifier R should be greater than 10.sup.13 .OMEGA.. In the case where the induction sensor is directly connected to the electrometric amplifier, it is not possible to have such an input resistance R as the existing amplifiers have the resistance of maximum 10.sup.9 .OMEGA. at the input, and the condition t&lt;&lt;.tau. is not fulfilled. This results in a substantial error in the measurement of the grain-size composition.